1. Field of the Invention
The invention relates to an apparatus for optically scanning a radiation-reflecting information plane, which apparatus comprises a radiation source supplying a scanning beam, an objective system for focusing the scanning beam to a scanning spot in the information plane and for re-imaging the scanning spot on a composite radition-sensitive detection system, and a first diffraction element arranged in the radiation path between the radiation source and the objective system for deflecting a part of the radiation reflected by the information plane to the radiation-sensitive detection system and for deforming the diffracted beam in such a way that a focus-error signal can be derived therefrom by means of the composite detection system.
A focus-error signal is a signal which is proportional to a deviation between the focusing plane of the objective system and the information plane.
The deflected sub-beam may be deformed by splitting the beam into, for example, two sub-beams each of which has a separate detector pair of the composite detection system associated with it. Another possibility is to render the beam astigmatic, which beam cooperates with four detectors arranged in four different quadrants in the detection system.
2. Description of the Related Art
An apparatus of this type, which is in principle suitable for reading information recorded in an optical record carrier and for optically inscribing such a record carrier, is known from U.S. Pat. No. 4,665,310. In this apparatus the composite diffraction element, in the form of a diffraction grating, fulfils two functions for which otherwise two separate elements must be used. In the first place the grating ensures that the radiation reflected by the information plane and passing through the objective system is defected from the path of the radiation emitted by the diode laser so that a detection system can be placed in the path of the reflected radiation. In the second place the grating splits the reflected beam into two sub-beams which are required for generating a focus-error signal, i.e. a signal comprising information about the magnitude and the direction of a deviation between the focusing plane of the objective system and the information plane. A separate detector pair is associated with each sub-beam, the difference signal between the output signals of the detectors associated with the same pair being a measure of focusing the scanning beam on the information plane.
In addition to a focus-error signal, a tracking-error signal must also be generated in the apparatuses considered. A tracking error is a deviation between the centre of the radiation distribution of the scanning spot and the centre of an information track portion to be read or a track portion to be written. In the apparatus according to U.S. Pat. No. 4,665,310 a tracking error is detected by means of the same detectors with which also a focusing error is detected.
To this end the diffraction grating in the known apparatus is oriented in such a way that the bounding line between two sub-gratings of which the grating is composed is parallel to the track direction at the area of the scanning spot. The tracking-error signal is obtained by determining the sum of the output signals of each detector pair and by subtracting these sum signals from each other. In the so-called single-spot system according to U.S. Pat. No. 4,665,310 deviations, or so-called offsets, may occur in the tracking-error signal, inter alia as a result of an asymmetrical energy distribution within the scanning beam. The tracking servo system which is used to keep the scanning spot centred on the track may be a so-called two-stage system. In such a system the coarse adjustment of the scanning spot is realised by displacing a slide supporting, for example, the radiation source and the objective system, relative to the tracks. The fine adjustment of the scanning spot is realised by displacing, for example the objective system relative to the slide. This displacement may cause a further deviation in the tracking-error signal. To prevent this deviation, or to compensate for it, extra measures have to be taken in practice. Moreover, when using the same detectors for generating a focus-error signal as well as a tracking-error signal, a focusing error may affect the tracking-error signal, or conversely, a tracking error may affect the focus-error signal.